Project Summary Respiratory tract infections are a leading cause of death worldwide. Compromised cell-mediated immune responses, low vaccine efficacy, and poor vaccine uptake all increase the likelihood that serious infection will ensue in pregnant women following exposure to respiratory viruses such as Influenza A virus (IAV); approximately 80% of pregnant women are at high risk for influenza-associated complications. Our best defense is vaccination. Vaccine-induced protection is mediated by immunological memory. The majority of immunological memory studies to date have focused on the differentiation, survival, and delineation of the antipathogen defense mechanisms employed by pathogen-specific T cells under normal physiological circumstances. The impact of pregnancy on the persistence and functional recall potential of memory T cells is surprisingly unknown. The overarching hypothesis of this proposal is that IAV-specific T cell memory generated prior to conception will mediate potent universal, heterosubtypic protection throughout pregnancy. This hypothesis arose from our observations that memory effector CD4 T cells isolated from influenza infected lungs express low levels of estrogen and progesterone hormone receptors in comparison to primary effector CD4 T cells. Pregnancy hormones are well known to have detrimental impacts on the generation and function of primary anti-viral immune responses. Preliminary findings herein show that heterosubtypic recall responses are indeed protective in gravid animals primed prior to conception. In the first aim, the persistence of IAV-specific CD4 and CD8 memory T cells during pregnancy, as well as their ability to differentiate into the many protective subsets that mediate diverse and potent anti-viral effector functions will be determined. Our prior work has shown that memory CD4 T cells regulate both local and systemic anti-viral inflammatory responses, and, additionally, mobilize innate lymphoid cells during pathogen infection. Innate lymphoid cells, which function in tissue repair, have recently been shown to be present in placental tissue. In the second aim, we will thus elucidate the role of IAV-specific memory T cells in preserving both healthy placental and fetal development through the regulation of systemic innate inflammatory and cellular responses following IAV infection. The insight gained from the proposed, `reverse-mechanism', studies will advance our ability to develop improved vaccines that elicit memory T cells capable of orchestrating protective anti-viral defenses during pregnancy. This research may also lead to the development of novel strategies to treat expectant mothers suffering from serious influenza for which current therapeutic options are lacking. Respiratory tract infection with Influenza A virus remains a serious public health concern, particularly for pregnant women, who have suffered disproportionally high morbidity and mortality in past epidemic and pandemic seasons. The proposed study has the potential to make a broad and significant translational impact on human reproductive health.